1. Field of the Invention
The present invention relates to an ink jet recording head drive apparatus and method thereof, which controls the diameter of an ink droplet to be ejected.
2. Description of the Related Art
In general, a recording head comprises a piezo-electric actuator to eject a liquid droplet for recording as necessary. Drop-on-demand-type ink jet recording heads are well-known, where a pressurized wave is generated in an ink chamber of the recording head by giving the piezo-electric actuator an electric signal, and with the help of the pressurized wave, a liquid droplet is ejected from a nozzle. Of these types of recording heads, an ink jet recording head is proposed (see Japanese Patent Application Laid-open No. Sho-51-37541, for example) where the diameter of a dot is changed in order to display a gradation image such as a picture image, enabling a gradation recording. FIG. 15 shows the ink jet recording head.
In FIG. 15, 181 denotes an ink jet recording head, 182 denotes a pressure chamber, and 183 denotes an ink supply layer. 184 denotes a first nozzle to connect pressure chamber 182 to the ink supply layer 183, 1000 denotes a diaphragm, 185 denotes a piezo-electric device, and 186 denotes a second nozzle.
When some pressure signal is applied on the piezo-electric device 185, a vibration is given to the pressure chamber 182 via the diaphragm 1000. The vibration causes generation of a pressurized wave in the ink in pressure chamber 182. The pressurized wave is then propagated to the first nozzle 184. The ink in the ink supply layer 183 receives the pressurized wave being ejected as an ink droplet 188 from a second nozzle 186.
When a gradation image is recorded using the ink jet recording head 181 operated in conformity with the above principle, the number of gradation levels L is represented by the following equation: EQU L=N.sup.2 (a)
where we assume that a single pixel is made up of a matrix of N.times.N dots 151 as shown in FIG. 17A, and therein the gradation is expressed by the arrangement of the dots 151 in the matrix. However, for an image which requires a high resolution and high gradation, such as that of a picture image, the size of the matrix N has to be larger, as is apparent from the above equation. Accordingly, the image configured with such pixels seems short in the resolution per a single pixel, and quite high dot-resolution is needed. Contrary to this, if the dot diameter is changed, each dot itself is allowed to have separate gradation levels. Thus, the number of gradation levels L is expressed by the following equation: EQU L=n.times.N.sup.2 (b)
where, n denotes the number of gradation levels per single dot. For the example, n=1, and N=3, shown in FIG. 17A, the number of gradation levels L is equal to 9, which is calculated using the equation (b). In contrast, as shown in FIG. 17B, if the dot diameter can change into one of four separate levels 151a to 151d the number of gradation levels L is thirty six due to the fact than n=4 and N=3 and the image configured with such pixels seems sufficient resolution per single pixel. Thereby, according to the above approach, without improving the dot resolution, the number of gradation levels can be increased. In this case where the dot diameter is controlled to vary, the volume Q of a ejected ink droplet is represented by the following relational expression: EQU Q.varies..tau..times.v.times.A (c)
Where, .tau. denotes the wave cycle of a pressurized wave generated in the pressure chamber 8, v denotes the ink droplet ejection speed, and A denotes the cross sectional area of a second nozzle 186. The ink droplet ejection speed, v has a relation as shown in the following expression: EQU v.varies.V (d)
Where, V denotes a voltage applied to the piezo-electric device 185.
According to the above expression, the peaks Pa to Pd of the pressure applied to the ink in the pressure chamber 182 differ dependent upon the increase/decrease of the applied voltage V, as shown in the pressure response chart of FIG. 16. The changes in the peak pressure Pa to Pd cause the change in the ink droplet ejection speed, v. However, due to the fact that the cycle .tau. of the pressurized wave does not change, the expression (c) only needs the parameter, the applied voltage V. therefore, the relation is represented by the following expression:
Q.varies.V (e)
In the conventional ink jet recording head, the relational expression is used to increase or decrease the volume of ink droplet 188 (Q) to be ejected from the second nozzle 186 by increasing or decreasing the applied voltage V to be applied to the piezo-electric device 185, and controlling the pressure P of the ink in the pressure chamber 182. However, in the approach where the ink droplet ejection speed, v is changed by the increase or the decrease of the applied voltage V, the flight speed of an ink droplet, to the relative speed of the head to the recording paper changes. Accordingly, the location on a recording paper where an ink droplet falls is slipped. This location slip degrades the recording quality. For example, ejection of a minute ink droplet may cause ink to easily fall around the second nozzle 186 due to the fact that the flight speed of the ink droplet is low.
In order to solve the above problems, as is shown in FIG. 15, in conventional approach, air-flow course 189 is added along the outside of the head. Therein, an air flow 191 is generated and flows out from a third nozzle 190 prepared in front of the second nozzle 186 at a constant speed, with the help of an air pump or an accumulator (not shown in the figure) prepared externally. The ink droplet 188 to be ejected from the second nozzle 186 is then carried along with the air flow 191. This configuration enables the successful control of increasing or decreasing the speed so that the speed can be equal to that of the air flow 191. However, this approach requires the attachment of the air pump or the accumulator, as a means to generate the air flow 191, to the head. Accordingly, preparation of an air-flow course is required in the body of the head. This creates a demand for a bigger, heavier, and more complex head.
The ink jet recording head, disclosed in Japanese Patent Application Laid-open No. Sho-61-100469, has been proposed as a means to solve the above problems. According to the proposal, while directing the attention to the above expression (c), the wave cycle .tau. of an ink pressurized wave is changed, and therefore the volume of the ink droplet to be ejected, Q is increased or decreased with the ink droplet ejection speed, v being constant. Specifically, several separate ink-flow courses with respective natural periods are installed so that pressurized waves with respective separate cycles .tau. are generated, and thereby that independent diameters of ink droplets are ejected from respective nozzles. However, there is a problem in having several ink-flow courses as a larger size of head which is high in cost is required.
In addition to that, as shown in FIG. 18, a wave with the wave forms in several natural oscillation modes is generated in the ink-flow course. A drop-on-demand-type ink jet recording head, disclosed in Japanese Patent Application Laid-open No. Sho-62-174163, for example, has been proposed in order to generate a specific oscillation mode. Wherein, with a piezo-electric device being attached on a single location or each of several locations of the loops in the amplitude of a wave form in the specific oscillation mode, the piezo-electric devices are driven.
As shown in FIG. 18(a), the part enclosed by a broken line shown in an ink-flow course 171 indicates the location of a piezo-electric device 172. As is shown in FIG. 18(b), the length of the piezo-electric device 172 is between the first node 176 and the second node 177 in the third-order natural oscillation mode 74 of the ink in the ink-flow course 171, whereas the location of the piezo-electric device attached, is the same as that of a loop 175 between the nodes. When a voltage wave, the wave form which fits the third-order natural period, for example, is applied to the piezo-electric device 172, a pressurized wave of comparatively short wave length in the third mode is generated in the ink in the ink-flow course 171. The pressurized wave generated causes for ejection with a comparatively small diameter of ink droplet. In order to generate a wave in high-order mode, for example fourth-order or fifth-order mode, it is recommended that a piezo-electric device is attached on the part corresponding to the loop in the oscillation mode, and that voltage is applied, the voltage form of which corresponds to the natural period.
However, in the conventional configurations, it is difficult to generate an oscillation mode other than both the basically natural oscillation mode (the first-order mode) and another higher oscillation mode. Therefore, a problem will occur where only two types of ink droplets in the basically natural oscillation mode and another higher oscillation mode, respectively, are obtained. Accordingly, it is difficult to obtain many independent dot-diameters, and to form an image with multi-gradation levels such as a picture image.